In any polymerization process, it is necessary to be able to control the molecular weight of the polymer produced so that it may be fitted to a particular use or need. For example, in unperturbed polymerization systems which fundamentally tend to produce high molecular weight polymers, it may be desirable or necessary to limit the molecular weight of the polymers produced, and this must be done in a fairly predictable and controllable fashion. Such molecular weight limitation may be desirable or necessary in the preparation of polymer solutions for use in paints and finishes which require high solids content to assure reduced solvent emission during application and yet which require low viscosity to facilitate ready application.
Controlling the molecular weight of polymers in an aqueous system has been especially limited or problematic. Mercaptans, such as butyl or dodecyl mercaptan, have been predominantly used to lower the molecular weight. However, in free radical polymerizations, there are a number of conventional means of effecting such molecular weight limitation. These, along with notable disadvantages or problems, include
(1) A high initiator/monomer ratio. However, this may be costly in terms of initiator consumption. Also, high initiator levels may also produce undesirable end groups on the polymers produced.
(2) Polymerization at high temperatures, particularly for suspension (bead) or emulsion polymerization. However, this may lead to undesirable depropagation, thermal initiation, and undesirable secondary reactions.
(3) Adding stoichiometric amounts of thiol chain transfer agents to the polymerizing system. However, the attendant incorporation of sulfur-containing agents into the polymer may render it less durable than is desired. There may also be odor problems associated with the use of sulfur-containing chain transfer agents.
(4) Chain transfer agents employing cobalt (II) chelates such as disclosed in U.S. Pat. No. 4,680,352 and U.S. Pat. No. 4,694,054, including aqueous and non-aqueous polymerization. However, a possible disadvantage of these is that some do not work in water and some are adversely affected or deactivated by low pH. They may cause color problems, especially if interaction with some monomers may require higher levels of the cobalt chelate.
The use of terminally ethylenically unsaturated oligomers or macromonomers as chain transfer agents as a means of controlling molecular weight of certain polymers in some contexts is known. There have been a number of studies and articles on macromonomers acting as chain transfer agents.
Macromonomers are known, for example, as disclosed in U.S. Pat. No. 4,547,323; U.S. Pat. No. 4,170,582; U.S. Pat. No. 4,808,656, Japanese patent 3,161,562; Japanese patent 3,161,593. See also, P. Cacioli, et al., J. Makromol. Sci.-Chem., A23 (7), 839-852 (1986) and H. Tanaka, et al., Journal of Polymer Science; Part A; Polymer Chemistry, 27, 1741-1748 (1989). Regarding the mention of aqueous polymerization, see U.S. Pat. No. 4,170,582 and Japanese Kokai Hei 3(1991)-161593 and Hei 3(1991)-161592.
It is an object of this invention to provide a method of polymerization in aqueous systems which employs a terminally unsaturated macromonomer as a catalytic chain transfer agent. It is a further object to control the molecular weight of the polymer or copolymer produced.
Use of the present invention avoids the use of sulfur bearing chain transfer agents with their associated problems. The present method also has a number of potential advantages, including lower polymerization temperatures, reduced initiator costs, and improved color. The resulting polymer and coatings may exhibit improved durability.
Yet another object is to provide an improved method of obtaining a final polymer based product less subject to ultraviolet degradability, making then useful in many applications, such as in paints and finishes. Other applications are in the area of imaging, electronics, for example photoresists, engineering plastics, and polymers in general.
These and other objects will become apparent hereinafter.